Genetic studies with Escherichia coli K-12 have demonstrated three alternative forms of a single regulator gene (capR) for polysaccharide synthesis and two more regulator genes (capS and capT) at other chromosomal loci. CapR, capS and capT mutants produce elevated quantities of several enzymes involved in polysaccharide synthesis and all three produce large quantities of the same capsular polysaccharide. The identical polysaccharide is also made in certain Salmonella species. Recent experiments indicate that one of the groups of enzymes elevated in capR and capT mutants, the galactose enzymes, is controlled by three types of regulator molecules, the capR repressor, the galR repressor, and the cyclic AMP system. The complexity of this system appears to provide an excellent model for differentiated systems that can be analyzed at the molecular level. CapR (also designated lon) but neither capS nor capT mutants are also very sensitive to UV and X-ray radiation and form filaments after radiation. At least eleven enzymes are involved in synthesis of the polysaccharide. Work is aimed at mapping the structural genes for these enzymes and measuring the enzyme levels in isogenic strains that contain different combinations of structural and regulator gene alleles. UV sensitivity of capR mutants does not require polysaccharide synthesis but is dependent on the state of the capR gene. Ochre suppressors that suppress nonsense mutations also suppress UV sensitivity in capR mutants. Such results, in addition to others, indicate that the product of the capR gene involved in UV sensitivity is a protein. Interspecies transduction experiments demonstrate that Shigella dysenteriae contains the wild type allele of capR. The relation between control of UV sensitivity and control of polysaccharide synthesis by capR alleles and the biochemical function of the capR product are being pursued.